Depressed 660-km discontinuity caused by akimotoite–bridgmanite transition

Artem Chanyshev (), Takayuki Ishii (), Dmitry Bondar, Shrikant Bhat, Eun Jeong Kim, Robert Farla, Keisuke Nishida, Zhaodong Liu, Lin Wang, Ayano Nakajima, Bingmin Yan, Hu Tang, Zhen Chen, Yuji Higo, Yoshinori Tange and Tomoo Katsura
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Artem Chanyshev: Deutsches Elektronen-Synchrotron DESY
Takayuki Ishii: University of Bayreuth
Dmitry Bondar: University of Bayreuth
Shrikant Bhat: Deutsches Elektronen-Synchrotron DESY
Eun Jeong Kim: University of Bayreuth
Robert Farla: Deutsches Elektronen-Synchrotron DESY
Keisuke Nishida: University of Bayreuth
Zhaodong Liu: University of Bayreuth
Lin Wang: University of Bayreuth
Ayano Nakajima: Tohoku University
Bingmin Yan: Center for High Pressure Science and Technology Advanced Research
Hu Tang: Center for High Pressure Science and Technology Advanced Research
Zhen Chen: Center for High Pressure Science and Technology Advanced Research
Yuji Higo: Japan Synchrotron Radiation Research Institute (JASRI)
Yoshinori Tange: Japan Synchrotron Radiation Research Institute (JASRI)
Tomoo Katsura: University of Bayreuth

Nature, 2022, vol. 601, issue 7891, 69-73

Abstract: Abstract The 660-kilometre seismic discontinuity is the boundary between the Earth’s lower mantle and transition zone and is commonly interpreted as being due to the dissociation of ringwoodite to bridgmanite plus ferropericlase (post-spinel transition)1–3. A distinct feature of the 660-kilometre discontinuity is its depression to 750 kilometres beneath subduction zones4–10. However, in situ X-ray diffraction studies using multi-anvil techniques have demonstrated negative but gentle Clapeyron slopes (that is, the ratio between pressure and temperature changes) of the post-spinel transition that do not allow a significant depression11–13. On the other hand, conventional high-pressure experiments face difficulties in accurate phase identification due to inevitable pressure changes during heating and the persistent presence of metastable phases1,3. Here we determine the post-spinel and akimotoite–bridgmanite transition boundaries by multi-anvil experiments using in situ X-ray diffraction, with the boundaries strictly based on the definition of phase equilibrium. The post-spinel boundary has almost no temperature dependence, whereas the akimotoite–bridgmanite transition has a very steep negative boundary slope at temperatures lower than ambient mantle geotherms. The large depressions of the 660-kilometre discontinuity in cold subduction zones are thus interpreted as the akimotoite–bridgmanite transition. The steep negative boundary of the akimotoite–bridgmanite transition will cause slab stagnation (a stalling of the slab’s descent) due to significant upward buoyancy14,15.

Date: 2022
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DOI: 10.1038/s41586-021-04157-z

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